Here is my personal opinion. I will go through each of the options and then make a 'ranking' at the end.
Paris: Personally, this would be my favorite place on an aesthetic level, but I'm not sure about it on an educational level. I agree with your description of it and I think it's very beautiful, but I also understand your criticisms. I think it depends on the university, it would be a generalization to say all of them don't care about their students, but I've also heard that the university atmosphere is incredibly independent. You have to figure out if you're up for that challenge; if you succeed, it would look even more impressive that you managed to achieve good grades almost entirely independently, but obviously if you're not a very independent learner it could be detrimental. Plus, the universities in Paris are usually considered quite prestigious, so it would definitely look good on a resume. Which school are you looking at?
When you say that 'you're just someone else in a big city', I understand what you're saying, but this is not necessarily the case. You just need to find your people, and then Paris becomes surprisingly cozy and small. I lived there for a summer, made a small group of friends with who I'd do everything with and it was amazing. If you don't manage to do this, there is a chance that you might feel a bit small in such a big city, but from a poetic view, it could be nice to lose yourself in a city.
Poland: Whilst I do love Poland, I'm not sure it would be a great place to study. I've never studied there so this isn't a first-hand experience, but from what I've heard the university system isn't great. I have a friend who moved to the UK from Poland and when I expressed my desire to travel there she told me I shouldn't. She said it's quite run down and as you said, their culture is quite closed off to outsiders. I don't know how true this would be to study there on an Erasmus year though since you will be surrounded by international students and I highly doubt you'll be going around the more run-down areas of the country. On the bright side, Poland is a fascinating place I've heard, and if you do manage to embrace their culture and get in with the locals it sounds spectacular!
Maribor, Bratislava, Lithuania, Latvia, Estonia, Budapest: I'm going to be completely honest, the only one I have heard of anyone studying in is Budapest. My step-sister did an Erasmus year there and she has nothing bad to say about it. She adored the city (and has taken the family there twice since she finished her year), loved the social/party atmosphere, thought the university really helped her progress, and just loved the experience in general. I can definitely understand where she is coming from because when I traveled there it was incredible. The city is so vibrant, the people are warm and inviting, there is always something to do and the architecture is stunning. The only thing I thought was sad was the extreme levels of poverty that I saw almost everywhere I went, it was shocking and upsetting as I couldn't help them all. But on the whole I'd probably say it's my favorite.
Spain: For Spain, I'd recommend studying in Valencia, as I think a larger city is always the best option when it comes to study abroad years. Once again, very inviting people and I've heard that the universities are good and the professors do their best to help the students. From what I've heard it is quite...independent though, kind of like with Paris. You're an individual in the country before you find your group of friends. It's a good destination if you like your warmth and beaches.
Croatia, Greece, Malta: For the exact reasons you put, I wouldn't recommend it. All beautiful countries but on an educational level, I'm not too sure if it's the best option.
Italy: You're right that the law schools are demanding, but an increase in grades isn't impossible. As with everything, it depends on how much effort you're going to put in. It does depend on what university you're looking at, but I think that Italian universities, on the whole, would look impressive to employers. Italian people are also welcoming, and I don't think you'll struggle to find a close group of friends. Italy can get pretty expensive though. I don't really have any more negatives about Italy.
Norway and Finland: Two of my favorite countries. Whilst they might be cold, yes, I don't think they're distant. The educational system, and pretty much all of their systems, are very efficient, and I think adapting to life there will be very simple. I think living there will help you to mature very quickly, but finding friends will be fairly easy. Their culture is so unique too, so that would be very interesting to explore in my opinion.
My overall ranking: Budapest, Norway, and Finland, Paris, Italy, Spain, Poland, Croatia, Greece, Malta.